1. Field of the Invention
The invention concerns timebases synchronized to an external clock, in particular those intended for use in digital telephone exchanges and more particularly the generators constituting these timebases.
2. Description of the Prior Art
It is conventional to use in telephone exchanges a central timebase delivering clock and synchronization signals to the various units through the intermediary of distribution means.
For reasons of security and reliability a timebase of this kind is of redundant design, comprising a plurality of identical generators, in most cases three in number.
In one known technique the generators utilize crystal-controlled oscillators with the frequency controlled by the application of a voltage and the temperature also controlled. Frequency and temperature control are obtained in the following manner:
the oscillator housing is thermostatically controlled to limit the effect of temperature changes, PA1 the oscillator operating range is fixed by the application of a calibration voltage to the control input, PA1 drift is controlled by superimposing a synchronization voltage and the calibration voltage, PA1 synchronization may be achieved by superimposing internal and external synchronization. PA1 a voltage-controlled oscillator, PA1 a first differential phase comparator having respective inputs connected to receive said external synchronization signal and said output signal of the timebase, PA1 a second differential phase comparator having respective inputs connected to receive said individual majority clock signal from said generator and an output signal from said voltage-controlled oscillator, PA1 a calibration system including a threshold detector connected to the output of said first comparator and a calibration voltage adjustment circuit connected to the output of said threshold detector, PA1 a summing circuit having its inputs respectively connected to the outputs of said calibration voltage adjustment circuit and of said first and second phase comparators and having its own output connected to a voltage control input of said voltage controlled oscillator.
Internal synchronization is achieved on the basis of the output signals of the oscillator itself and of the oscillators of the other generators of the timebase, by means of a majority decision logic circuit having inputs connected to receive the output signals from the generators. External synchronization is achieved by means of a link delivering an external clock. Internal synchronization must take precedence over external synchronization.
A timebase of this kind is described in French Pat. No. 2 390 856, corresponding to U.S. Pat. No. 4,164,629.
In use, the generators are thus mutually synchronized, but their power supplies and their means for distributing clock signals to the other units must be completely independent. For each utilization there is provided a distribution module comprising a majority decision logic circuit which receives the output signals from the generators.
A signal distribution system of this kind is described in French patent application No. 81 17643, corresponding to U.S. Pat. No. 4,489,412.
The known technique has disadvantages given the specifications currently applicable to telephone exchange timebases, particularly in the case of synchronized networks.
The generator frequency must be set with greater accuracy and must be very stable. Synchronization to an external clock calls for extremely fast correction of any phase shift, and may only vary the frequency within very narrow limits.
Fast phase control is needed in the event that the external clock may itself be subject to phase shifts, in the case of cut-off or switchover to a back-up clock, for example.
The object of the invention is to provide a timebase in which each generator provides a highly stable frequency, achieved through automatic control of the oscillator calibration, and the phase of which relative to an external clock can be modified very quickly.